Maternal serum is screened during the second trimester for elevations in alpha-fetoprotein, which may indicate that the fetus has an open neural tube defect. alpha-Fetoprotein is a glycoprotein synthesized in the fetus, and it appears in the amniotic fluid and maternal serum. With an open neural tube defect involving the fetus, AFP can show increased leakage into the amniotic fluid and maternal serum. If an elevated maternal AFP relative to the gestational age is detected, more definitive tests are performed in order to find the cause for the elevation.

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